1. Field of the Invention
The present invention relates to flameless emission spectroscopy and, more particularly, to a method of and apparatus for generating nitrogen for use in flameless emission spectroscopy.
2. Description of the Prior Art
A variety of methods and systems exist for the quantitative and qualitative detection and analysis of atomic and molecular species in samples, such as body fluids. One such method and system involves the excitation of the sample atoms through collision with an active metastable gaseous species in a Lewis Raleigh afterglow, the excited atoms then emitting characteristic wavelengths of light as they relax back to their ground state. Specifically, the sample to be analyzed is introduced into a gas stream containing an excess of an active metastable species of nitrogen or other hoble gas whereupon the material, if atomic, is rapidly and repeatedly excited or, if molecular, is decomposed and certain component atoms of the molecule are excited, the excited species emitting characteristic wavelengths of light. The wavelength and intensity of the emitted light are determinative respectively of the identity and concentration of the atoms of the different elements present.
There are at least two advantages to this flameless emission technique. The first of these is that upon relaxation to the ground state, the atoms may collide again with an active nitrogen molecule, providing that the active nitrgen is present in excess, thereby reexciting the atom with a subsequent reemission of a characteristic photon. The second advantage is that in contrast to flame photometry or atomic absorption photometry or other techniques, the background radiation in the Lewis-Rayleigh afterglow region is extremely low in the visible and ultraviolet, permitting the characteristic emission spectra to be observed against a black background. These two advantages combine to make this technique extremely sensitive and capable of extremely good linearity. As a result, a number of systems have been developed utilizing this technique.
The generation of active nitrogen is accomplished in a microwave discharge. Specifically, the technique requires the injection of gaseous nitrogen into a microwave cavity coupled to a microwave generator as well as the mixing of the active nitrogen stream with the sample stream in the vapor phase. A suitable sample introduction system for generating the sample stream in the vapor phase is disclosed in copending patent application Ser. No. 922,938 filed concurrently herewith, entitled Sample Introduction System for Flameless Emission Spectroscopy, and assigned to Beckman Instruments Inc., the assignee of the present application. The sample introduction system of that application, as well as those of other systems, require nitrogen in gaseous form which is typically received from a gas bottle. While providing the gas in this form generally is acceptable, there are circumstances when it is not. Particularly for clinical applications, it is not desirable to have a tank of nitrogen present, simply because of the danger associated therewith. For this reason, clinicians have been reluctant to utilize the flameless emission technique, in spite of its many advantages.